Male pond skaters forced to 'sing' for sex

作者：吕讠 发布时间：2019-03-08 07:20:01

By Andy Coghlan (Image: C. S. Han and P. G.Jablonski/PLoS ONE) (Image: C. S. Han and P. G.Jablonski/PLoS ONE) Video: See the serenading skaters By evolving a “shield” for their genitals, female pond skaters can decide for themselves whether to allow males to mate with them. They decide whether or not to mate by listening to “songs” made when male suitors use the tips of their long mid-legs to tap out rippling rhythms on the water surface. In other pond skater species – also known as water striders – males run the show, mating with females almost at will because they have no means to resist. Nor is there usually any form of courtship. But the red-backed water strider, found in stationary pools beside mountain streams in Korea, Japan and China, has now proved to be the exception. Detailed experiments and anatomical analyses have now shown that females of Gerris gracilicornis have evolved a protective shield for their genitalia. Moreover, they will only lift the shield if males go through an elaborate mating ritual lasting about 15 minutes, in which they use the tips of their mid-legs to tap out the intricate rhythms. The experiments were carried out by Piotr Jablonski and Chang Han of Seoul National University in South Korea. What they found is that the penultimate abdominal segment of the female has grown to cover three quarters of the final segment, which contains the genitalia. In almost all other pond skaters, it covers half the final segment or less, offering zero protection against marauding males. “In this species, the female is in control because she normally keeps her genitalia hidden behind a shield of the last abdominal segment,” says Han. “Only when the female expands and protrudes her genitalia from behind this shield can the male have access for mating.” “This is exactly the kind of thing you might expect to evolve in a system where females are vulnerable to forced matings,” comments Tim Birkhead of the University of Sheffield, UK, who in 2007 published evidence that female mallard ducks have intricate vaginas allowing them to send unwanted sperm into a cul-de-sac. “The authors are aware that there are no winners in such arms races, but at any point in time, one sex may be ahead of the other – and here, it seems that the female water striders are ahead,” says Birkhead. To win their prize, courting red-backed strider males must tap out three consecutive songs. Firstly, the males grip the female from behind and manoeuvre her into the mating position with their forelegs. During this ritual, they extend their long mid-legs forwards and begin tapping rhythms – dubbed “grasping signals” – on the water. Next come the “mounting signals”, during which the male engages his own genitalia with the partially exposed genitalia of the female. Finally come the most regular bouts of tapping, when the female finally yields and lifts the shield, exposing her ovipositor and allowing full mating to occur. Jablonski and Han demonstrated that the tapping is vital for mating by disrupting the movements with a tiny metal bar fitted to the backs of the females. This was wide enough to prevent the males reaching the water with their mid-legs, putting a stop to their songs. The team says that the courtship tapping is almost certainly a male counter-strategy to rediscover lost influence in the mating game. One possibility is that the quality of the rhythm reveals the fitness of the male, enabling the female to mate only with the best available. It may be that the female has internal anatomy that enables her to accept or reject the sperm after mating, and the serenading helps her decide which choice to make. Journal reference: PLoS One (DOI: 10.1371/journal.pone.0005793) More on these topics: